The present invention relates to an inclination monitoring system, which is used, for example, in a lens inclination adjustment apparatus.
Conventionally, a data recording/reproducing device using an optical disk as a recording medium is provided with an optical system which includes a light source and an objective lens held in a lens holder. A recording medium (i.e., an optical disk) is set at a focal point of the objective lens. The light source emits a parallel light beam which is focused by the objective lens to form a beam spot on the recording medium. The beam reflects on the recording medium, and is received by an optical pick-up device.
In the data recording/reproducing device, an optical axis of the objective lens preferably intersects the recording medium at right angles. If the optical axis is inclined with respect to a normal line to the recording medium, coma occurs, which may cause the beam spot formed on the recording medium to be wider than it should be. If the beam spot is wider, the density at which data is recorded to the recording medium will be lower. Further, if the beam spot is wider, when recorded data is read, a reproduced signal may include noise.
Recently, an optical system having a larger numerical aperture NA has been used for such a device. In this case, even a small inclination of the objective lens may greatly affect the size of the beam spot on the recording medium.
Accordingly, it is necessary to adjust the inclination of the objective lens such that an inclination angle formed between the optical axis of the objective lens and a line normal to the recording medium is within a permissible range, which is, for example, 3 minutes of arc.
In order to adjust the inclination of the objective lens, a lens inclination adjustment system is used. A conventional inclination adjustment system includes an interferometer unit and a lens inclination adjustment unit.
Firstly, at least a part of the data recording/reproducing device, including the optical system and the light source, is coupled with the interferometer unit and interference fringes are observed. A user may then determine the inclination angle and inclination direction (i.e., a direction of the inclination on a plane parallel to the surface of the recording medium and facing the objective lens) based on the observed interference fringes.
Next, at least a part of the data recording/reproducing device is coupled with the lens inclination adjustment unit and the lens is moved so that the inclination (i.e., the inclination angle and inclination direction) is adjusted to be in a permissible range in accordance with the inclination angle and the inclination direction which have been determined using the interferometer.
When the lens is moved (i.e., when the inclination of the lens is changed), it is necessary to monitor the change of the inclination, or current inclination, of the lens. An example of an inclination monitoring system employed in an inclination adjustment apparatus is shown in FIG. 1.
Generally, an objective lens 1 is molded and includes a lens portion 2 and a planar flange portion 3 surrounding the lens portion 2. Generally, monitoring of the inclination of the lens 1 is performed by emitting a light beam to the flange portion 3 and detecting the reflected beam.
As shown in FIG. 1, the monitoring system includes an He--Ne laser source 404 and a screen 407. The lens 1 is positioned approximately one meter away from the He--Ne laser source 404 and the He--Ne laser source 404 emits a narrow light beam P, having a diameter of 1-2 mm, towards the flange portion 3 of the lens 1 through an opening 409 formed on the screen 407. A reflected beam P', reflected by the flange portion 403, is incident on the screen 407. It should be noted that the flange portion 3 may be formed as a mirror surface to improve reflectivity.
The reflected beam P' is observed as an image on the screen 407, and a center of the image is regarded as a point where the optical axis of the reflected beam intersects the screen 407. The inclination of the lens 1 is then monitored with reference to the position of the center of the image formed by the reflected beam P' on the screen 407.
However, since the objective lens is made from a mold, the surface of the flat portion 3 is microscopically uneven and the image of the reflected beam is not a perfect beam spot. As shown in FIG. 2, the image may extend over a wide area, such as, in this example, approximately 26 minutes of arc. Accordingly, the center of the image of the reflected beam is difficult to identify, especially if the inclination of the lens 1 is changed during adjustment.
Further, part of the beam directed to the flange portion 3 may be incident on the lens portion 2, and a reflected beam from the lens portion 2 may form another spot or stray light on the screen 407, making it even more difficult to adjust the inclination of the lens 1.
FIG. 3 is an enlarged view of the lens portion 2. The lens portion 2 includes a first surface 2A, on which the laser beam P may be incident, and a second surface 2B, opposite to the first surface 2A. An optical axis O1 of the lens 1 and the first surface 2A intersect at a point 2C.
As shown in FIG. 3, depending on the diameter of the beam P, parts P1 and P2 of the beam P may be incident on the surface 2A at different distances from the optical axis O1. For this analysis, we assume that the beam P is parallel to the optical axis O1, i.e., such that an inclination angle .phi. of the beam P with respect to the optical axis O1 is zero. In fact, in the conventional inclination monitoring system, since the inclination of the lens 1 is to be adjusted, the beam P may form a small angle with the optical axis O1.
As shown in FIG. 3, the part P2 of the incident beam P is incident at an incident height (a distance between the optical axis O1 and the position where the part P2 is incident) h=1.7 mm. A portion of the part P2 travels through the lens portion 2, is reflected by the inner surface of the second surface 2B six times (total reflection), and is then emitted from the first surface (as stray light) at an emitting angle (an angle of the emitted beam with respect to the optical axis O1) .PSI. of approximately -3.0 degrees. Similarly, for the part P1 of the beam P, inclination angle .phi.=0 and incident height h=1.6 mm, such that a portion of the part P1 is reflected by the second surface 2B three times and emitted from the first surface 2A at an emitting angle .PSI. of approximately 30 degrees.
FIG. 4 is a graph showing a relationship between the incident height h of the incident beam and the emitted angle .PSI.. Note that, in this graph, the optical axes of the objective lens 1, the light monitoring device, and the incident beam are assumed to be parallel with each other. In the graph, a numeral in parentheses represents a number of times a portion of an incident beam is reflected by the second surface 2B. Thus, from the graph, it may be seen that a beam which is incident to the first surface 2A at an incident height between 1.14 mm through 1.46 mm is totally reflected twice by the second surface 2B and is emitted at an emitting angle .PSI. between approximately 48 through -65 degrees. Further, a beam which is incident to the first surface 2A at an incident height of approximately 1.5 mm through 1.6 mm is reflected three times and is emitted at an emitting angle .PSI. between approximately 30 through -40 degrees. Note that the emitting angle .PSI. will vary according to the angle formed between the optical axis of the monitoring device and that of the lens 1 (i.e., depending on .phi.). Thus, the beam reflected by the second surface 2B may not be substantially attenuated and may be directed to the screen 407 as stray light, making it difficult to monitor the inclination of the lens 1. The above values are for a lens 1 having the following characteristics: a radius of curvature of the first surface 2A of approximately 4.5 mm; a radius of curvature of the second surface 2B of approximately -2.8 mm; a refractive index of approximately 1.54; and an effective diameter of approximately 3.3 mm.
As described above, it may be difficult to monitor the inclination of a lens precisely due to stray light caused by the lens itself.